The term "flux" used throughout the specification applies to a solid, gas or liquid agent that promotes solder wetting and good solder joints through removal of surface oxides on the metallic surfaces to be soldered. Traditional flux used with lead based solder is a liquid flux, however, flux may also be applied in powder or granular form, or in a gaseous form such as a reducing gas to reduce surface oxides. If soldering occurs in an atmosphere which is substantially oxygen free, then in some cases the quantity of flux required for good solder wetting may be reduced.
In the past, flux has generally been applied by flux applicators wherein the circuit boards or elements to be solder coated or solder joined have flux applied thereto prior to passing through a solder wave.
In U.S. Pat. Nos. 5,044,542 and 5,121,874 to Deambrosio et al a shield gas is applied about a solder wave and the more conventional type of fluxes need not be used. Instead the shield gas may contain a reducing agent which acts as a flux to prevent oxidization on the surfaces to be solder wetted. In these patents, the shield gas may be applied by diffusers positioned on both sides of the solder wave below the conveyor path and over the solder wave above the conveyor path.
In U.S. Pat. No. 5,048,746 to Elliott et al is disclosed a reducing gas atmosphere which includes a small quantity of hydrogen, insufficient to provide an explosive mixture. The term "fluxing" referred to in U.S. Pat. No. 5,048,746 may not specifically include a reducing gas, however, the term "fluxing" used in the present application is to include any agent that promotes solder wetting and this could be a reducing gas or other agent such as adipic acid and the like.
In U.S. Pat. No. 5,090,651 to Mittag a tunnel is provided through which elements pass on a conveyor. A solder wave is disclosed in the tunnel. The tunnel has gas curtains and in at least one embodiment, reference is made to an additive included with the gas curtains so that elements passing into the tunnel prior to soldering are contacted with an additive. This additive may be a flux in solid, liquid or gaseous form, the solid form being a powder or in granular form, a gas can include a reducing agent gas to provide a fluxing action, and in another embodiment adipic acid additive or other acidic compounds in powder or liquid form are disclosed as being added at the gas curtains. In the tunnel, the atmosphere is controlled generally to provide an atmosphere which is substantially oxygen free. The flux may be entrained, sprayed, atomized or the like, directly into the gas supply forming the gas curtain so it is applied to the elements passing into the tunnel.
In U.S. Pat. No. 5,203,489, the disclosure of which is incorporated herein by reference, is disclosed a shroud placed around a solder wave having a slot for the solder wave to project therethrough and provision is made for gas to be provided under the shroud through gas diffusers to blanket the solder wave with a gas that substantially excludes oxygen and preferably is an inert gas such as nitrogen.
The patent discloses supplying gas through the gas diffusers and states that the type of gas may include a reducing gas and may include additives which are desirable for solder coating of wettable metallic surfaces or for joining at least two wettable metallic surfaces.
In U.S. Pat. No. 5,240,169, the disclosure of which is incorporated herein by reference, is disclosed shrouds placed around solder waves with diffusers positioned under the shrouds to blanket the solder wave with shield gas. The patent discloses a fluxer to add flux to the elements prior to preheating and application of solder. Therefore there is no suggestion in this patent that a reducing agent or other additive be included with the shield gas. The patent also shows a gas knife positioned after the solder wave. The gas knife is used to blow off any excess solder on the element after it exits from the solder wave. The gas used is a shield gas or may be air. Clearly the gas knife is not used for the addition of flux or a reducing gas.
In U.S. Pat. No. 5,292,055, the disclosure of which is incorporated herein by reference, is disclosed yet a further embodiment of a shrouded solder wave.